2 Al 2 O 3 Thin Film Deposition Using Thermionic established in vacuum condition in the vapors of the material to be deposited [10, 11]. MgO thin film deposition [12] and carbon deposition [13] using TVA have been reported. A TVA system at Osmangazi University in the year of 2000 was constructed [14]. The aim of this paper is to present the preliminary results of Al 2 O 3 TVA depositions. 2. EXPERIMENTAL DEVICE The TVA consist of a directly heated cathode, which is a tungsten filament mounted inside a molybdenum Wehnelt cylinder, and an anode which is a tungsten crucible containing the materials to be evaporated. A photograph of the cathode used in the present experiment is shown in Fig. 1. Figure 1. Photo of the cathode showing the filament placed inside the Wehnelt The two electrodes are mounted on a table which is placed in the center of a vacuum cylindrical stainless steel chamber having a volume of 65 lt. A schematic presentation of the TVA electrode arrangement for an interelectrode angle 0 o < ϕ < 90 o is shown in Fig. 2. The vacuum was obtained by a conventional pumping system, which consists of a rotary pump with a capacity of lt/s, and a diffusion pump with a capacity of 135 lt/s. The pressure was measured by an active pirani gauge with range of atmosphere to 10-3 mbar and an active inverted magnetron gauge with range of 10-2 to 10-9 mbar connected to an active gauge controller. The electrical system consists of two power supplies (low voltage and high voltage), a voltmeter (0-1800V), an ammeter (0-2A), and a ballast resistor (300Ω). The thermoelectrons emitted by the cathode are focalized by the Wehnelt cylinder, and accelerated toward the anode containing the material to be evaporated (Al 2 O 3 pellets) by the applying high voltage. The accelerated electron beam, incident on the anode, heats the crucible together with its content to high temperature, Al 2 O 3 starts to evaporate and a steady state density of the evaporated Al 2 O 3 is established in the interelectrodic space. For a convenient density of Al 2 O 3 molecules and decomposed atoms and a needed value of the applied voltage, a thermionic 19

4 Al 2 O 3 Thin Film Deposition Using Thermionic As seen in Fig. 3, at the beginning, the characteristics are identical with those of vacuum diodes, but at an increased value of the applied voltage a sudden jump of the current is observed, simultaneously with the ignition in vacuum conditions of a bright Al 2 O 3 TVA discharge between electrodes. The ignited arc current for fixed electrode geometry and fixed cathode heating current is constant as can be seen in the volt-ampere characteristic of Al 2 O 3 TVA discharge. If the cathode filament heating current increases in the ignited arc, ignited arc current increases. This variation obtained in ignited Al 2 O 3 TVA discharge is given in Fig. 4. Dependence of the ignited arc current of Al 2 O 3 TVA discharge on the filament current is similar to dependence of that of the metal vapor TVA discharges on the filament current. This is important because TVA discharge can be controlled by the filament current Al 2 O 3 Iarc (ma) I f (A) Figure 4. Dependence of the ignited arc current on cathode filament heating current The thin films of Al 2 O 3 have been deposited on the glass substrates of 10x20 mm size with a thickness of 1mm. The surfaces of the deposited Al 2 O 3 thin films were analyzed by a MOM. MOM images obtained various points of our samples are given in Fig. 5. glass Al 2 O 3 thin film (a) Figure 5. (a) MOM image of border of glass- Al 2 O 3 thin film, (b) MOM image of midpoint of Al 2 O 3 thin film (b) 21

5 T. Akan, E. Karakaş, G. Musa Sigma 2005/3 The surface morphologies given to Fig. 5 prove the smoothness of our Al 2 O 3 thin films. Black points on the micrographs belong the glass surfaces as can be seen in Fig. 5a. MOM images of various points of our sample and the samples deposited various discharge currents are similar to Fig. 5. Also the smoothness of the Al 2 O 3 layer is proved by the obtained SEM images given in Fig. 6. Figure 6. SEM photograph of the Al 2 O 3 deposited film Thin Al 2 O 3 films were analyzed by X-ray diffraction (XRD) method. XRD analysis result of an Al 2 O 3 films are presented in Fig SiO 2 Al 2 O 3 (112) Intensity (a.u.) MgAl2O4 C SiO2 C θ Figure 7. X-ray diffraction spectrum of the Al 2 O 3 thin film. The XRD spectrum of the Al 2 O 3 thin film shows the dominant orientation is (112). The TVA discharge produces energetic neutral atoms besides energetic ions with directed energies. The energy of ions can be controlled by TVA voltage drop [15] and changed at will even during deposition, achieving even more than 400 ev [14]. This means that on the substrate arrive not 22

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